Under certain circumstances, it may be desired to retard a machine's propulsion. For example, when a machine is travelling down an incline it may be necessary to retard the machine's propulsion in order to maintain a desired speed. In another example, during propulsion direction change events (e.g., forward to reverse) it may be desired to retard the machine's forward propulsion before propelling the machine in the reverse direction. Typically, the retarding event is initiated by adjusting the transmission (e.g, downshifting) to drive power from the traction members (wheels) back into the engine. The natural resistance of the engine then dissipates some of the transferred power, thereby retarding the machine. However, driving the power into the engine typically causes the engine speed to increase. When the engine speed increases above a maximum desirable engine speed, the engine's capacity to absorb power is limited to steady state frictional losses of the engine. As a consequence, the retarding rate of the machine decreases, requiring more time and a greater distance for the machine to slow down to the desired speed.
In order to maintain a retarding rate, accessories may be actuated to enhance engine dissipation capabilities. One such method is described in U.S. Pat. No. 6,330,873 (“the '873 patent”) issued to Letang et al. on Dec. 18, 2001. During the retarding event, the system disclosed in the '873 patent dissipates power by increasing the exhaust stroke pressure of at least one of the cylinders to effect four degrees of engine braking which correspond to zero, minimum, moderate, or maximum power dissipation, respectively. When maximum power dissipation is requested, the system also increases engine accessory load by enabling a cooling fan in addition to modifying the exhaust stroke pressure. This allows the system to better control vehicle speed when descending long grades.
While the '873 patent may disclose a method to enable a cooling fan in order to achieve maximize engine power dissipation, improvements to engine retarding capabilities may be limited. In particular, the disclosed system of the '873 patent does not modulate the cooling fan based on engine dissipation parameters.
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems in the existing technology.